The invention concerns a control device for hydraulic variable displacement pumps which are adjustable on one side according to the generic concept of claim 1 and a variable displacement pump fitted with such a control device according to claim 10. In particular, the invention concerns hydraulic variable displacement pumps which are operated in an open hydraulic circuit and which are adjustable by means of a servo piston that can shift inside a servo cylinder, to which, in turn, pressure can be applied via a control device by means of pressurized pressure fluid. For this purpose, the servo piston acts on a displaceable adjustment element or transmission component, for example the swash plate or the bent axis, thereby adjusting the angular position of the latter and thus also the displacement volume of the variable displacement pump according to the position of the control piston in the control device. The variable displacement pump is preferably configured as an axial piston machine in swash plate or bent axis design, whereby the inventive concept can also be applied to radial piston pumps or vane pumps insofar as the latter are adjustable in their pump pressure via a servo control unit.
The invention is described based on variable displacement pumps that are adjustable on one side by means of a servo control unit, such pumps demonstrating maximum flow rate when the servo control unit is set to zero pressure or power. However, the inventive concept also covers the reverse construction of this type of variable displacement pump, in other words also variable displacement pumps which demonstrate minimum flow rate, i.e. are deflected to a minimum extent, when the servo control unit is set to zero power. If the servo control unit does not exert any force on the displacement volume adjustment element, the variable displacement pumps used as examples to explain the inventive concept are moved into a position of maximum deflection, usually by means of internal springs or similar which act on the adjustment element. This means that when the servo control unit applies a force to the adjustment element of the variable displacement pump, the latter is deflected out of its maximum position in the direction of reduced output. In the case of minimum flow rate, i.e. minimum high pressure, the variable adjustment pump is set to a minimum deflection angle. The servo control unit then exerts a maximum force on the adjustment element of the variable displacement pump, for example a swash plate or a bent axis.
A control device for such variable displacement pumps is familiar from DE 199 49 169 A1. This document describes a variable displacement pump configured as an axial piston pump with a deflectable swash plate and operated in an open hydraulic circuit. The adjustment device for the angular position of the swash plate comprises a servo piston which shifts inside a servo cylinder to which pressure can be applied via pressurized liquid by means of a control device, the control device being safeguarded by means of an additional pressure control valve in order to limit maximum operating pressure. The control device comprises a control piston with two control edges that is mounted in a housing so that it shifts longitudinally. The housing of the control piston comprises an inlet for fluid under high pressure from the variable displacement pump, an outlet which can be connected to a tank and a hydraulic connection that is linked to an inlet of the servo cylinder. A link can be created between the inlet and the connection via a first control edge, and a second control edge can be used to create a link between the connection to the servo cylinder and the outlet to the tank. The control device according to DE 199 49 169 A1 is complex in structure and comprises a large number of elements including several springs and a solenoid which acts directly on the servo piston.
A disadvantage in the control device shown in DE 199 49 169 A1 is that it requires a large number of components and an additional, elaborate pressure control valve placed between the outlet to the tank and the housing of the control piston, and that the control piston and servo piston interact via a spring. In addition, it involves adjustment of the neutral position of the servo piston via an eccentrically mounted disc which forms an end stop for a valve sleeve.
In DE 195 38 649 A1 a pump regulation valve is controlled by means of double-sided application of hydraulic pressure to the control piston. Here a pressure upstream of a consumer directional valve exerted on a first side of the control piston acts against a pressure tapped downstream of the consumer directional valve which is exerted on a second side of the control piston. The displacement volume of the variable displacement pump is set based on the different in pressure in the connection lines by means of a setting device.
The invention is therefore based on achieving the object of creating a control device for hydraulic variable displacement pumps adjustable on one side of the type described above which are simple and robust in construction and easily adjustable. The control device according to the invention shall make do with a small number of components and allow an even increase and decrease in the flow rate of the variable displacement pump during operation while still retaining load-dependent power regulation of the variable displacement pump. It should be possible to set various power levels for the variable displacement pump which can be reliably maintained by the automatically regulating control device according to the invention without requiring any external control intervention. In addition, the control device according to the invention should not require additional pressure control valves in order to limit maximum high pressure.